State of Technology Report for Force Main Rehabilitation, Final ...

Figure 6-1. Polyurethane Pipeline Cleaning Pigs
(Photo courtesy of Pipeline Pigging Products)
High Pressure Water Jetting
Jetting has become very common, yet is limited in its capabilities. It is currently the most popular
cleaning method for gravity sewers and is widely used throughout the world. Jetting machines range
from compact van-pack units for cleaning small diameter drains, through trailer-mounted jetters of
various power ratings, to full tanker-jetters (which may also combine vacuum removal) up to the largest
unit of a water recycling facility.
Although high pressure jetting can achieve excellent results if used wisely, it also has the potential to
make a bad situation worse and to create problems where none existed. Jetting a cracked or fractured
pipe may cause the fragments to become loose and fall into the pipe, creating a collapse and a blockage.
Persisting with high pressures in an attempt to remove the blockage can wash away the pipe surround,
resulting in further destabilization and perhaps even subsidence at the surface. Leaving a high pressure
jet in one position for more than a few seconds may puncture the pipe or damage the wall, even if the
pressure is below the recommended maximum.
There is some risk in jetting at high pressure in plastic pipes, notably uPVC. Generally, the wall
thickness of a solid wall uPVC pipe is such that high pressure jetting is unlikely to cause serious damage,
although excessive pressures should be avoided. The main concern is with structured wall polymeric
pipes where most of the strength is provided by external ribs or corrugations, and the inner wall can be
quite thin (sometimes less than 2 mm). Structured wall pipes were introduced to reduce the required
quantity of material (and hence the weight and cost) for medium to large diameters, while providing
adequate stiffness to resist external loads.
The main drawback of high-pressure jetting is that the debris in the sewer can be flushed downstream
rather than being removed. Jetters may be excellent for clearing localized blockages, but are less well
suited to dealing with large volumes of silt or similar material over long lengths of sewer.
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